1. Field of the Invention
The present invention relates to a semiconductor laser device for use as a light source for optical communication and so forth, and more specifically to a semiconductor laser device having such structure as capable of preventing COD (catastrophic optical damage) and improving operational reliability.
2. Description of the Prior Art
A semiconductor laser device for use as a light source for optical communication and so forth needs to have high operational reliability and to hardly exhibit change in characteristics for a long time. For example, a semiconductor laser device used in a repeater for an optical fiber submarine cable needs to keep operational reliability for over a million hours.
Types of failure (degradation in operational characteristics) of the semiconductor laser device are broadly classified into gradual degradation, which is analogous to fatigue failure and due to long-time operation, and rapid degradation, which is due to COD produced at facets under high-power operation. The gradual degradation in operational characteristics is mainly caused by such a phenomenon that crystal defects inherent in the semiconductor laser device gradually increase as DLD (dark line defects), which may lead to failure of the semiconductor laser device. Causes of the gradual degradation in operational characteristics, however, have been mostly removed with development in semiconductor manufacturing technology. On the other hand, causes of COD produced at the facets of are still in process of analysis. At present, it is considered that repetition of the following process causes COD: When crystal defects existing at the surface of the facets of the semiconductor laser device absorb an emitted laser beam, the temperature at the facets rises, so that a band gap at the facets is reduced; as a result, the crystal defects increase.
In order to prevent the COD in the semiconductor laser device, such structure has been conventionally proposed that a non-absorbing layer of, for example, InGaP having a band gap greater than the band gap of an active layer of the semiconductor laser device is formed on the facets of the semiconductor laser device.
As shown in FIG. 3, a conventional semiconductor laser device of the aforementioned sort comprises, as its main part, a semiconductor laser main body 5 of double hetero structure in which, for example, an active layer 4 of InGaAs sandwiched between a pair of cladding layers 2, 3 of n-AlGaAs and p-AlGaAs is grown on a substrate 1 of n-GaAs. Facets 6 and 7 of the semiconductor laser main body 5 are formed by cleaving, and a non-absorbing layer 8 of InGaP, which has a band gap greater than the band gap of the active layer 4 and is optically transparent to a laser beam, is formed on one of the facets 6, 7 (an exit side cavity end face 7). Further, a dielectric protective layer 9 of AlO.sub.x, SiO.sub.x, SiN.sub.x or MgO.sub.x, which is for regulating reflectance at the facet 7 and protecting the non-absorbing layer 8, is formed on the non-absorbing layer 8. Reference numeral 10 indicates a reflecting film of laminate of SiO.sub.2 and Si, having high reflectance and formed on the other facet 6. The thickness of the non-absorbing layer 8 is 80 nm or so, and the thickness of the dielectric protective layer 9 is 180 nm or so.
In the semiconductor laser device of the above described structure, since photo-absorption due to the crystal defects at the facet 7 can be restrained by the non-absorbing layer 8 of InGaP, COD can be prevented to a certain degree. However, there remains a problem that COD is still produced when the semiconductor laser device is made to operate with high power or operate continuously for a long time.